O2 pay-as-you-go customers get charged £2 per day for up to 25MB of data outside the UK. Go over the 25MB and you can pay another £2.

Vodafone will charge £8 for up to 100MB per day.

Orange offers several data bundles (that's their page for using your phone in Germany) -- 30MB for £3 per day, or three monthly offers: £15 for 30MB over a month, £50 for 150MB over a month or £150 for 500MB over a month.